1. Field of the Invention
The present invention relates to a vehicle frame structure in which an upper edge of a frame member having an upwardly opening U-shaped cross-section is fixed to a lower face of a substantially flat panel so as to form a closed cross-section, and the frame member has a curved portion formed in a longitudinally middle section thereof, the curved portion having an upwardly curving bottom wall.
2. Description of the Related Art
If a longitudinal load due to a collision of an automobile is applied to a front side member having a closed cross-section that is bent in a crank shape when viewed from the side, there is a problem that a bending moment is generated in the bent portion (kick-up portion), thus causing a large degree of bending deformation. Japanese Patent Application Laid-open No. 2003-231483 discloses an arrangement in which the kick-up portion of the front side member is reinforced by a reinforcing member only on the compressive deformation side, thereby improving the rigidity of the kick-up portion while minimizing the increase in weight.
Also, a portion of an automobile rear frame mounting a rear wheel suspension system is curved upward into an arch shape in order to avoid interference with a suspension spring or a hydraulic damper, leading to another problem that a floor panel covering an upper face of the rear frame should also be curved upward, thus hindering effective utilization of the capacity within a vehicle compartment. If the upper face of the rear frame is made flat in order to make the floor panel flat, not only is the rigidity greatly degraded by an arch-shaped curved portion formed on a lower face of the rear frame, but also the curved portion is bent upward by the longitudinal load imposed on the rear frame when the automobile is involved in a collision, whereby it becomes difficult to exert an impact absorbing effect by making a front or rear portion of the rear frame effectively buckle.
In order to solve these problems, the arch-shaped curved portion formed in the rear frame is reinforced by a reinforcing member, thus making it difficult for the curved portion to be bent by the longitudinal load. However, unless the shape and arrangement of the reinforcing member are carefully determined, only the weight is increased without increase of the strength against the bending load.